action

Action, in physics, is a quantity with the dimensions of energy × time. It is most commonly
encountered in modern physics in connection with Planck's
constant, which has these dimensions and is therefore sometimes called
Planck's quantum of action. Pairs of measurable quantities whose product
has dimensions of energy × time are called conjugate quantities in quantum
mechanics, and have a special relation to each other, expressed in Heisenberg's
uncertainty principle. In classical physics, the term originated in
the 17th century and assumed a variety of different meanings to do with
the evolution of a system between two defined states. It crops up in the
statement of Newton's third law "Action and reaction are equal and opposite,"
although the interpretation of this is unclear unless "action" is defined.